1. Field of the Invention
The invention relates to a tensioning device for the skins of percussion instruments, composed of several bearing brackets which are provided with threads and are arranged distributed over the circumference of the instrument shell, and screws which are adjustable in the bearing brackets and act on a hoop for holding the skin. By means of a clamping piece of an elastomer material which surrounds the tensioning screw, the screws can be secured automatically in each tensioning position relative to the bearing bracket or the threads in the bearing brackets.
2. Description of the Related Art
A tensioning device of this type has become known from DE 41 06 492 A1. The clamping piece has an opening through which extends directly the thread of the screw or a threaded sleeve receiving the screw, wherein the smallest width of the opening is below the external diameter of the thread of the screw or of the threaded sleeve. As the material for the clamping piece, all those materials having rubber-like elasticity can be used which are available for the technical application. The clamping pieces can be manufactured particularly of natural rubber, synthetic rubber, rubber-like materials or also other synthetic materials.
Since the smallest width of the opening--which may be circular or may have any other contour shape--is never greater than the external diameter of the thread at the shaft of the screw, it is achieved that the thread of the screw shaft penetrates into the walls of the opening while the material of the clamping piece is elastically deformed. As a result, the thread is subjected to a constant breaking force which has the purpose of preventing an automatic or undesired turning of the screw relative to the tensioning device. A non-rotating abutment for the clamping piece may be formed by the bearing bracket and/or directly by the shell body or the like of the percussion instrument which supports the bearing bracket. Accordingly, the breaking force acting on the screw is produced exclusively by the fact that the thread of the screw engages with elastic deformation of the clamping piece in the walls of the opening.
In a tensioning device known from DE 28 13 883 C2, the screws are provided at their shafts with at least one flattened or indented circumference portion which can be placed within the threaded bushings or sleeves in a circumference portion of a spring stirrup with radial pretensioning and, thus, forms with the flattened or indented circumference portion a turning and lock device between the screw and the threaded sleeve. This manner of securing the position of the tensioning device not only offers a self-locking effect in any chosen position of rotation between the screw and the threaded sleeve, but it additionally produces a positively locking effect in certain relative positions between screws and threaded sleeves. However, this requires a relatively complicated construction and, therefore, results in a high expenditure in the manufacture of percussion instruments.